1. Technical Field
This invention concerns materials which can serve as intermediate agents for transfer of heat from an energy source to a load object. More specifically, it relates to granular compositions of matter which are particularly suitable for cooling applications.
2. Description of the Related Art
In a typical heating application, agent Y receives heat from source X and then delivers its stored heat to load object Z. The same applies, in principle, to a cooling application. In that case, agent Y receives cold from (gives up its heat to) source X and then delivers its cold to (receives heat from) load object Z. In other words, one may consider the load object of cold as a source of heat and the source of cold as the load object of heat. Similarly, cold may be considered as a deficit of heat or negative heat. While intermediate agents of heat transfer play a like role in heating and cooling, this invention focuses upon the latter.
Water is commonly used as a cooling agent because it is readily available, inexpensive and entirely safe to use in its liquid or frozen state. In terms of functional properties, it also possesses high specific heat and high heat of melting/freezing. Thus, heat withdrawn from water as it is cooled and frozen, is ultimately reabsorbed by the water when it is used as a cooling agent, en route back to ambient temperature. The total cooling effect is the sum of sensible heat absorbed by ice and water plus the latent heat consumed by the process of melting.
Familiar examples of cooling applications, with water, include ice cubes chilling a beverage and perishables preserved in crushed ice. They demonstrate the basic elements necessary for effective cooling; namely, direct contact and substantial immersion of the cooling object in the cooling medium. The concurrent effects of these cooling applications are not undesirable. In the first case, melted ice mingles integrally with its cooling object and becomes part of the beverage. In the second, surface water from the melting ice promotes heat transfer and keeps the cooling objects rinsed fresh and clean.
There are, of course, many cases where direct contact between cooling medium and cooling object are not desirable. Various methods of water containment have begin devised for such applications. They include, ice bags, freezer packs and the like. Most of them comprise water, aqueous compositions or gels of either, hermetically sealed in a rigid or flexible, ostensibly leak proof container, sized and shaped for its intended use. These forms of containment essentially preclude immersion of cooling object in a cooling medium, a subject which will be revisited later. In addition, there is always some risk of damage to the containers and loss of cooling medial.
Other methods of containment involve the disposition of water into small, liquid absorbent, solid particles. This approach retains tile desirable thermal properties of water, albeit somewhat diluted and proportionally diminished by the solid carrier. It eliminates the dangler of water leakage per se. However, it still requires appropriate containment of the solid particles for most practical applications. It also requires vapor tight containment to prevent loss of water by evaporation.
To better understand the present invention in light of prior art, it is important to make a clear distinction between conformability and free flowing properties of a particulate solid. The latter may otherwise be referred to as flowability. Any aggregate of discrete particles in a flexible container is conformable; i.e. it may be bent and shaped manually to fit around another object. Flowability, on the other hand, does not necessarily require manipulation. It comes into play under the influence of gravity, as discrete particles tumble and flow freely, when contained loosely, to surround any underlying object. Free flowing properties are particularly important in a healthcare application, where heating or cooling pads need to drape around parts of the body.
In U.S. Pat. Nos. 5,211,949 and 5,282,994, Salyer teaches the absorption of water, among other phase change materials, into submicron particles of silica. The particles range in size from 0.005 to 0.25 microns, typically 0.022 microns. The resulting composition contains about 60-70% water and 40-30% silica, or a payload of 150% to 233% water based on the weight of the silica substrate. This powder-like formulation is claimed to be surface dry, comfortable and free flowing down to freezer temperature, properties highly desirable for cooling applications. Cooling media of this prior art are indeed conformable as claimed. However, because of their light bulk density and fine, dust-like character, they do not possess that decided gravitational flowability which is most desirable for healthcare applications. Moreover, the dusty character of this medium preclude heating or cooling applications which might involve direct immersion of a load object into the heat transfer medium.
The use of discrete particulate heating/cooling agents is disclosed by this applicant in U.S. Pat. Nos. 4,937,412, 5,314,005 and 5,417,276. The preferred substrate in that technology consists of activated alumina in the form of spherical beads ranging in size from 1/32" to 1/8" in diameter. The beads contain microwave responsive payloads, predominantly non-volatile, which are absorbed in the beads and retained therein by capillary forces. Uniformity of shape and surface smoothness, combined with high bulk density, produce flowability ideally suited for healthcare applications. These particulate media were intended mainly for heating pads, which only incidentally doubled for cooling. That precluded payloads composed primarily of water, sine the latter would evaporate and be lost in successive heat applications.
A subsequent development program of the same technology attempted to capitalize on the thermal properties of water as a phase changing payload solely for cold applications. Beads of activated alumina were loaded with pure water and the resulting composition appeared to be surface dry and high in bulk density, with potential flowability highly desirable for healthcare cooling applications. Unfortunately, these desirable free flowing properties did not survive storage in a freezer in preparation of the beads for experimental cooling applications. Evidently moisture reaching the surface of the beads resulted in a solid frozen mass.
The prior art has addressed similar problems, i.e. particle freezeup due to surface moisture, by coating the particles with liquid hydrocarbons or by using water soluble additives to weaken ice which does form. This invention takes an entirely different approach. It employs finely divided, hydrophobic solids dispersed between the particles to disrupt the buildup of integral ice formations which would otherwise bind the particle together into a frozen mass.
Accordingly, the object of this invention is to propose granular heat transfer media dedicated solely to cooling applications. The proposed media take full advantage of water as change-of-phase payload, with its free flowing properties preserved from ambient to sub-freezing temperatures.